A NOx purification system using a selective reduction type NOx catalyst device has been put in practical use in order to process NOx (nitrogen oxide) in an atmosphere of excessive oxygen in a process for the purification of an exhaust gas from a diesel engine. This NOx purification system provided with the selective reduction type NOx catalyst device includes: a selective reduction type NOx catalyst (SCR catalyst) device for selectively reducing NOx by using ammonia (NH3) as a reducing agent; and as an ammonia supply source, an ammonia-based solution supply unit for adding an ammonia-based solution, such as an aqueous urea solution, to the exhaust gas, which is installed in a stage before, i.e., at an upstream side of the selective reduction type NOx catalyst device.
The aqueous urea solution ((NH2)2CO) added to the exhaust gas from this ammonia-based solution supply unit is hydrolyzed and thus produces ammonia through a reaction represented by Reaction Formula (1) “(NH2)2CO+H2O→2NH3+CO2.” The ammonia thus produced is used as a reducing agent in the selective reduction type NOx catalyst device to purify NOx. This selective reduction type NOx catalyst device reacts ammonia and NOx with each other through reactions represented by Reaction Formula (2) “4NH3+4NO+O2→4N2+6H2O,” Reaction Formula (3) “2NH3+NO+NO2→2N2+3H2O, ” and Reaction Formula (4) “8NH3+6NO2→7N2+12H2O.” Thereby, the selective reduction type NOx catalyst device reduces NOx to nitrogen.
It is considered that: among these reactions, the reaction represented by Formula (3) progresses more easily than the reaction represented by Formula (2) and the reaction represented by Formula (4) under a low temperature; and the reaction represented by Formula (3) progresses most easily when a ratio (molar ratio) of NO (nitric oxide) to NO2 (nitrogen dioxide) is 1:1, i.e., when a proportion (molar ratio) of NO2 in NOx is 50%. Nevertheless, almost all of NOx contained in the exhaust gas discharged from the engine is NO. With this taken into consideration, employed is a method of enhancing the capability to purify NOx under a low temperature by installing an oxidation catalyst device, which oxidizes part of NO contained in the exhaust gas to NO2, at an upstream side of the ammonia-based solution supply unit.
In relation to this, for instance, Japanese patent application Kokai publication No. 2007-154819 proposes the following method of enhancing the capability to purify NOx under a low temperature. This method makes a ratio of NO to NO2 in the exhaust gas, which flows into an SCR catalyst (selective reduction type NOx catalyst), closer to 1:1 as much as possible, which is advantageous for the purification of NOx under the low temperature. To this end, the method causes part or all of an exhaust gas G discharged from an engine to be supplied to the SCR catalyst (selective reduction type NOx catalyst) after causing the part or all of the exhaust gas G to pass a NO oxidation catalyst (oxidation catalyst), and causes the remaining part of the exhaust gas G to be supplied to the SCR catalyst through a bypass passage which does not allow the remaining part of the exhaust gas G to pass the NO oxidation catalyst. In this respect, with consideration being given to conditions and the like of the exhaust gas and the capability of the NO oxidation catalyst in oxidizing NO, the method enhances the capability to purify NOx under the low temperature by: controlling the flow rate of the exhaust gas G to be allowed to pass the NO oxidation catalyst and the flow rate of the exhaust gas G to be supplied to the SCR catalyst without being allowed to pass the NO oxidation catalyst; and thus making the ratio of NO to NO2 closer to 1:1. In addition, Japanese patent application Kokai publication No. 2007-154819 proposes a method of preventing the deterioration of the NO oxidation catalyst by allowing no exhaust gas G to pass the NO oxidation catalyst when the temperature of the exhaust gas G is high.
However, the adsorptivity of NO2 produced through the oxidation using the oxidation catalyst is far higher than that of NO. For this reason, until the adsorption of NO2 reaches saturation, NO2 produced by using the oxidation catalyst is adsorbed to the oxidation catalyst. As a result, only unreacted NO is supplied to the selective reduction type NOx catalyst, but no NO2 is supplied to the selective reduction type NOx catalyst.
On the other hand, if the temperature rises after the volume of adsorbed NO2 reaches saturation, NO2 is released due to decrease in the saturation volume of NO2 adsorbed to the oxidation catalyst along with the temperature rise. For this reason, an unexpectedly large volume of NOx is supplied to the selective reduction type NOx catalyst.
Accordingly, the ratio of NO to NO2 and the volume of NOx which flows into the selective reduction type NOx catalyst vary largely depending on how much of NO2 is adsorbed to the oxidation catalyst. Thus, the estimation of the ratio of NO to NO2 with the method of the conventional technique which gives no consideration to the adsorption of NO2 is inaccurate. Consequently, the conventional method cannot achieve the object of enhancing the rate of NOx purification by controlling the ratio of NO to NO2.
Variation in the ratio of NO to NO2 results in variation in the activity in purification, and accordingly leads to variation in an amount of ammonia to be consumed, namely a necessary amount of ammonia-based solution such as an aqueous urea solution. For this reason, it is highly likely that ammonia slip occurs due to the excessive amount of ammonia-based solution to be added, and that the capability to purify NOx decreases to a large extent due to the insufficient amount of ammonia-based solution to be added.
Patent Document 1: Japanese patent application Kokai publication No. 2007-154819